We explored how systematic manipulation of the physical characteristics of the Kanizsa and Ebbinghaus illusions influences perceptual outcomes in the general population. Additionally, we investigated whether these effects on illusion processing are influenced by individual differences in autistic traits. Eighty-five adults aged 18 to 35 years completed the Autism Spectrum Quotient (AQ) and participated in two illusion tasks in which the size, spacing, and direction of inducing elements were systematically manipulated. In addition to accuracy analysis, performance was assessed using the Balanced Integration Score (BIS), a metric that integrates accuracy and reaction time.Our results showed that, in the Kanizsa illusion, both the orientation of the illusory shape and the ratio of inducer size to spacing (i.e., support ratio) influenced perception. Similarly, in the Ebbinghaus illusion, both the size ratio of the two central circles (i.e., center ratio) and the relative size of the surrounding versus central circles (i.e., outer ratio) affected illusion strength. Autistic traits did not show consistent or robust modulation of the effects of physical stimulus characteristics across tasks. These findings suggest that illusion processing strength in the general population is primarily determined by stimulus geometry. Our findings suggest that predicted differences between within-object and between-object illusions with respect to autistic traits may not be evident in non-clinical samples.
Mashayekhi et al. (2026) studied this question.